The present invention relates to semiconductor memory technology. More particularly, the present invention relates to electrically programmable read only memory (EPROM) devices and to ones of those devices known as "flash" EPROMs, and to methods for operating such devices.
2. The Prior Art
A great deal of interest has been displayed recently in flash EPROMs. Flash EPROMs are memory devices which are programmed by injection of channel hot electrons to a floating gate and erased by tunneling of electrons from the floating gate to the substrate. The charge on the floating gate affects the current flow through the channel underlying the floating gate. This current can be sensed to determine the charge on the floating gate and, hence, the state of the memory bit.
A nonvolatile memory must have a large difference in the voltages used during programming and erasing and those used during reading in order that the data storage really be nonvolatile, that is that the leakage currents induced by the fields present during sensing be so much lower than those currents induced during writing that the charge is maintained for tens of years. This is usually accomplished by applying a voltage or voltages greater than 5 volts to the memory cells which are to be programmed or erased.
It is very desirable to have an array of flash EPROM cells which would operate entirely from the 5 volt power supply commonly found in modern digital systems. It is common today for a memory circuit to operate from a single 5 volt power supply where the higher programming and erase voltages are generated on chip by charge pumps. On-chip charge pumps are relatively inefficient and incapable of supplying even moderate amounts of current. It is therefore required that the nodes to which these charge pumps are connected draw little power.
For various reasons, the prior art has been unable to satisfy the need for a method for programming and erasing a flash EPROM cell which is suitable for use in a high density EPROM memory array and which permits complete operation of such an array from a single 5 volt power supply. There currently exists a need to provide such a method.